The nuclear power industry utilizes aqueous solutions within the reactor side of the nuclear reactor to transfer energy from the reactor core to the electrical generators. These solutions often become radioactive and must be treated to reduce the level of radioactivity. Generally, the solutions are processed through mixed ion exchange beds containing both cationic and anionic resin material. The resin material contains anionic and cationic exchange sites and these sites are termed "active" if they have not been replaced by an ionic moiety, such as a radioactive species, and "exhausted" if they have been replaced by an ionic moiety. Commonly, the resin bed material is removed from service prior to the complete exhaustion of the resin material. This partially exhausted ion exchange resin must then be disposed of as a radioactive waste, which wastes are highly regulated by governmental agencies. Particular problems associated with disposal of ion exchange media include the need to remove excess residual water while maintaining the resin in a "swollen" or wet state for long term burial, and the nature of the resin material which may still be partially active.
The nuclear waste industry has developed various ways for disposing of radioactive ion exchange resin. Problems associated with its disposal are related to its capacity for further ion exchange and its consistency as resin beads which are sand-like in nature. Examples of disposal techniques include incineration, solidification in concrete, confinement of the resin beads in a high integrity container, and solidification in polymeric materials. Ultimately, these treated wastes are buried in regulated burial sites.
Incineration is generally not preferred due to its equipment requirements that translate into high operating costs. Also, incineration produces secondary radioactive wastes, such as fly ash and off-gases, that require additional treatment. Further, the high sulfur content of the resin beads has presented problems with damage to the incinerator vessel.
Solidification with concrete is undesirable due to the increase in waste volume and weight. Further, the resin beads tend to interfere with the concrete chemistry and cause weaknesses in its structural integrity.
The use of high integrity containers is in many ways satisfactory, however the ion exchange resin is in a loose state and can be easily released into the environment in case the container vessel is damaged. The resin can also escape if a fire should damage the container.
In the recent past, considerable interest has been focused on the area of resin treatment that results in its solidification with polymeric binders. Examples of earlier methods are shown in U.S. Pat. No. 4,077,901 to Arnold et al. and U.S. Pat. No. 4,167,491 to Gablin et al. in which the waste ion exchange material is stirred into the polymeric binding material within a containment vessel. The stirring is continued for a short period of time after which the stirring means is discontinued and the polymer allowed to cure within the vessel. This system is disadvantageous in that the entire mass of waste material and polymeric binding material must be admixed prior to curing of the polymer. Since the polymer curing is an exothermic reaction, a potentially dangerous situation arises where the contents of the container could reach an unsafe temperature, and even lead to a fire. If less polymer catalyst, or curing agent, is added to try to control the exotherm, with the optional introduction of additional curing agent after initial mixing, the resulting solidification process may not be uniform, there is still the chance for an uncontrolled exotherm upon the introduction of additional curing agent, and there may not be adequate time for mixing due to the pot life of the polymeric mixture. This method also produces an undesired secondary waste in the form of the mixing apparatus, which could be left in the container as shown in Gablin et al., which is not economical.
Another processing method is to first admix the waste ion exchange resin and polymeric materials in a blending vessel and subsequently transfer the contents to another containment vessel. This process is shown in Gablin et al. and also commercially used in the French SETH 200 process available from Technicatome. The ion exchange material is first placed into the vessel and then the polymeric reagents are blended into that material using conventional mixing techniques. The mixer is then removed and the polymer allowed to cure. As discussed with the other method, this method produces an entire ion exchange/polymer resin mass before the exothermic curing reaction. Such a situation is dangerous due to the possibility of an uncontrollable exothermic reaction. These processes also do not lend themselves well to relatively large disposal vessels due to the presence of the large mass of potentially dangerous reactant polymeric material.
The efforts of later development work centered around perfecting these "in-containment" processes in which the ion exchange media and polymeric material were both placed into one containment vessel for both mixing and storage. One embodiment was developed in which the ion exchange resin was first added into the containment vessel and the polymeric binding material was introduced into the top of the vessel while a vacuum was pulled from the bottom of the vessel. The polymer and its curing agent were thus drawn through the void spaces in the ion exchange resin until the polymer mixture contacted the vacuum orifice. The polymer mixture was chosen to be hydrophobic and would displace the residual water that is hydrated on the surface of the swollen ion exchange media. This process also had problems. First, the concept required that the polymer and its curing agent could transit the entire length of the disposal vessel through the tightly packed resin bed within the pot life of the polymer system, which was not always the case. Second, the activated polymer mixture was observed to be reactive with the ion exchange resin. Essential reactive species from the polymer mixture were removed during transit through the resin material; thus the polymer mixture reaching the bottom of the vessel was not in proper stoichiometric proportion. It has been found that trying to pretreat the resin bed to exhaustion to remedy this situation resulted in the liberation of radionuclide species creating a radioactive secondary waste. Also, trying to anticipate a loss of reactive species by overloading the curing agent would lead to unacceptable exotherm conditions. Finally, the polymer mixture, upon being pulled through the resin matrix, would tend to be channeled through certain passages causing incomplete matrix formation.
A need therefore exists to develop a process for the encapsulation of waste radioactive ion exchange resin that can be accomplished within the containment vessel itself without the need to mix the entire mass of ion exchange resin and encapsulating polymeric material prior to the curing of the polymeric material. A further need exists for a process that effectively preconditions the ion exchange resin prior to the encapsulation process so that undesired reactions between ion exchange media and the encapsulating polymeric material does not occur.